1. Field of the Invention
The present invention relates generally to heat pumps and devices and systems that effect the temperature of the surrounding area or a chosen medium, and in particular to a monitoring and control system and method for a heat pump, preferably an air source heat pump, such as an air source heat pump used in connection with a swimming pool, aquarium, fish pond, or other body of water or liquid.
2. Description of the Related Art
Air source heat pumps have been used in various applications to remove heat from the outdoor air and move it to another fluid or heat sink for space and water heating, as well as other applications, such as process heat for industrial and commercial applications, swimming pools, agricultural aquariums, fish ponds, and the like. Such heat pumps are increasingly being utilized in applications where a cost effective heating method is required, such as in areas where the fossil fuel cost per BTU (British Thermal Unit) delivered is greater than the cost of the electricity required to move a BTU of heat from the air using a heat pump.
However, in certain low temperature air source heat pump applications where the exhaust air temperature (i.e., the air already cooled by the heat pump's evaporator) moves below the freezing point, the moisture it contained is left behind frozen on the evaporator tube. In this manner, subsequent and severe icing can occur in the heat pump and its components, which impacts and limits the ability of the heat pump to effectively extract heat from air. When the air temperature goes below the approach of the heat pump evaporator, thus causing the moisture in the air to freeze on the evaporator tubes, the frost effectively insulates the refrigerant inside from further heat transfer.
In a space heating application, where heat must be delivered to the living space in a structure, a second more-costly heat source is often required, such as fossil fuel or resistance electric heat, in order to provide space heating below the heat pump icing temperature. Where the same space heating heat pump is designed using a reversing valve to also provide air conditioning or cooling in warm temperatures, it is capable (in winter heating mode) of defrosting the evaporator or outside coil by operating temporarily in the reversed mode, and removing heat from the heated space to melt the ice on the outside coil. While this is more efficient than using more costly backup heat, at some lower air temperatures or certain applications, the cost advantage is lost, and backup heat must be used for economical reasons.
In the case of air-to-water heat pumps, such as swimming pool heaters, where this limitation is less critical and where frosting only occurs at the low end of the “swimmable” or required range of operating air temperatures, a backup source or costly reversing valve can be avoided if the heat pump's frost-free low air temperature operation is maximized. The actual air temperature at which icing occurs in any outdoor evaporator coil is a function of the tube surface temperature, the amount of moisture in the air, and the air velocity. Although any moisture present will condense below the dew point, and deposit ice or frost as the tube surface temperature is below freezing, the amount of moisture in the air determines how much frost or ice will accumulate and how fast.
In such applications, there exists a cut-off air temperature, below which icing is certain at almost any humidity, and slightly above that is an air temperature at which negligible frost will occur regardless of the humidity. The tube surface temperature will reach freezing at a range of internal refrigerant evaporating temperatures, depending on the mass flow rate of the refrigerant, as well as the size and design of the heat exchanger and the air velocity. To avoid ice formation by shutting off the heat pump as it reaches the freeze point, existing heat pumps either directly measure the tube surface temperature or the air temperature, and shut off the heat pump at an appropriate temperature above the frosting point.
While measuring the tube surface temperature appears to an effective straightforward method, it requires a sensor on the tube, which, in turn, requires wiring and electronic circuitry (or a thermostatic bulb type sensor and capillary controller), all of which significantly increase the cost of the heat pump controls and reduce it's reliability, since the additional sensor and wires (or capillary and bulb) are subject to damage from weather, rodent, insect, or human tampering. In addition, when ice does form on or around the sensor, it will affect the sensors accuracy until the ice melts, which can delay restarting the heat pump when the air temperature has risen well above the frost point, thus defeating the intended purpose of maximizing low temperature operation.
An alternate method to address this frosting issue is to indirectly monitor the refrigeration system pressure (using a pressure switch) to determine the refrigerant evaporating temperature inside the tube. The switch is set to cut-out at a pressure corresponding to a tube surface temperature that is at the freezing point.
Using a pressure switch is a less costly alternative, as most heat pumps already use one to detect loss of refrigerant, and this switch could be set to a cut-out pressure corresponding to an evaporating temperature, which produces a tube surface temperature just above freezing. Once the switch shuts the heat pump off, the suction or evaporating pressure and the condensing or discharge pressure will equalize, since the compressor has stopped pumping gas. This stabilization pressure is equal to the saturation pressure of the refrigerant gas/liquid mixture at the ambient air temperature, which will be higher than the suction pressure. Thus, the cut-out pressure of the switch must be set at the suction pressure corresponding to freezing tube surface temperatures when the heat pump is running, while the reset or cut-in pressure of the switch must be slightly above the off-cycle stabilization pressure of the heat pump when it is off and equalized, otherwise the heat pump will constantly cycle off and on.
The accuracy of the pressure switch method, and using the refrigerant pressure to determine the frost cut-out temperature, is directly dependent on assumptions regarding the calculation of the tube surface temperature, and how much the pressure measured directly reflects that value. This approach is also subject to variations due to refrigerant over- or under-charge, as well as equipment performance variations, including the tolerances on the switch cut-out pressure and cut-in pressure.
Further, currently available low-cost pressure switches depend on an internally-mounted, snap-action, dome-shaped metal disc that collapses or pops out to trigger the on and off set points. Such a design is inherently inexpensive, but also has a fairly wide built-in hysteresis, which is the difference between cut-in and cut-out pressure. This design also has a widely variable pressure trip point tolerance. These factors limit the pressure switch's ability to have a cut-out pressure close to its cut-in or reset pressure, such that once the switch turns off the heat pump, it cannot reset until a much higher suction pressure and air temperature than necessary is reached, thereby losing additional low air temperature runtime. The cut-out pressure is also a function of the air flow and the overall system design, such that a different cut-out pressure is required for each model or different size heat pump, which complicates manufacturing and design, moreover increasing the cost.
Existing Industry experience using the pressure switch method has been problematic, especially with the added variability introduced by the mandated use of a specified refrigerant blend (R410A), and its glide characteristics, which cause the saturation pressure for a given refrigerant or air temperature to be variable and unpredictable. In the past, refrigerant R22 was used and had a very predictable saturation temperature for a given refrigerant or suction pressure.
As an alternative to using a fixed pressure set point switch for each model variation, measuring the actual refrigerant pressure may eliminate the use of multiple switches with different set points, since a commonly used microcomputer controller could be programmed to use the appropriate pressure for each heat pump model and design variation. However, such an approach requires a pressure transducer and attendant electronics and wiring, which is also subject to the same damage as the temperature sensor method previously discussed. In addition, the cost of a pressure transducer is relatively high, which is why current heat pumps have not adopted their use.